1. Field of the Invention
The present invention relates to a piezoelectric transformer and a method for manufacturing the same, and more specifically to a laminated piezoelectric transformer advantageously incorporated in a small-sized rectifying power supply which is required to have a small size, a light weight and high reliability, and a method for manufacturing the same.
2. Description of Related Art
In the prior art, a solenoid type electromagnetic transformer was used as a step-down transformer in a so-called AC adapter for supplying an electric power to various battery-cell-driven electronic instruments from a power-frequency voltage distribution system. The electromagnetic transformer is composed of a magnetic core and a conducting wire wound around the magnetic core by a large number of turns. Because of this construction, it is actually difficult to realize a small-size, light-weight electromagnetic transformer.
On the other hand, a piezoelectric transformer utilizing a piezoelectric effect, which is completely different from the electromagnetic transformer in operation principle, has been provided.
The piezoelectric transformer is discussed in for example C. A. Rosen, "Ceramic Transformer", Proc. of Electronic Component Symposium, 1957. In this connection, various mounting or packaging methods for the piezoelectric transformer have been proposed in the prior art (See for example, Japanese Patent Application Pre-examination Publication Nos. JP-A-08-274382 (an English abstract is available from the Japanese Patent Office and the content of the English abstract is incorporated by reference in its entirety into this application) and JP-A-08-298213 (which corresponds to U.S. Pat. No. 5,747,916, the content of which is incorporated by reference in its entirety into this application).
Now, an example of the Rosen type piezoelectric transformer which is a typical prior art piezoelectric transformer, will be described with reference to FIG. 1A which is a diagrammatic perspective view illustrating the prior art Rosen type piezoelectric transformer mounted on a support member, and which corresponds to FIG. 13 of JP-A-08-274382, and also with reference to FIG. 1B which is a diagrammatic sectional view taken along the line A--A in FIG. 1A but showing a condition that the piezoelectric transformer is supported within a housing.
Referring to FIGS. 1A and 1B, the shown prior art Rosen type piezoelectric transformer includes a piezoelectric ceramic plate 510 in the form of an elongated plate, which is divided into a driver section 51 and a generator section 52. The driver section 51 is polarized in a thickness direction of the piezoelectric ceramic plate 510, and therefore includes a pair of planar electrodes 511 and 512 formed on upper and lower surfaces of the piezoelectric ceramic plate 510, respectively, so as to substantially cover the whole of the upper and lower surfaces in the driver section 51. On the other hand, the generator section 52 is polarized in a lengthwise direction of the piezoelectric ceramic plate 510, and an end electrode 515 is formed on the generator section side end surface of a pair of end surfaces in the lengthwise direction. Furthermore, support members 520 are provided each to surround the piezoelectric ceramic plate 510 along a direction orthogonal to the lengthwise direction of the piezoelectric ceramic plate 510.
The piezoelectric transformer having the above mentioned construction is used for a voltage step-up. Now, an operation principle of the piezoelectric transformer will be described.
If an AC voltage is applied from an external between the upper and lower planar electrodes 511 and 512 of the driver section 51, namely, between a pair of input terminals 517 and 518 connected to the planar electrodes 511 and 512, respectively, the driver section 51 is caused to vibrate in the lengthwise direction because of a piezoelectric lateral effect, in accordance with the magnitude of the applied AC voltage.
As a result, lengthwise direction vibration occurs in the piezoelectric ceramic plate 510, so that because of a piezoelectric longitudinal effect of the vibration, a stepped-up voltage having the same frequency as that of the input voltage is generated between the planar electrode 511 or 512 of the driver section 51 and the end electrode 515 of the generator section 52 (in the example shown in FIG. 1A, between the planar electrode 512 and the end electrode 515).
Next, a packaging of the above mentioned Rosen type piezoelectric transformer will be described. Referring to FIG. 1B, in order to support the piezoelectric transformer vibrating in the lengthwise direction because of the piezoelectric lateral effect, it is important to support the piezoelectric transformer without attenuating the vibration. In ordinary cases, the support member 520 is positioned in a region which becomes a node of the vibration called a ".lambda. mode".
In this prior art example, the support member 520 is formed of an elastic material such as rubber. Namely, two elastic members 520 formed of rubber are mounted to surround the piezoelectric ceramic plate 510 at vibration node positions, and an outer surface of the elastic members 520 is fixed to an inner surface of a housing 521 which accommodates therein the piezoelectric ceramic plate 510. Thus, the piezoelectric ceramic plate 510 is packaged.
Now, another structure of the package of the Rosen type piezoelectric transformer will be described with reference to FIG. 2A which is a diagrammatic perspective view illustrating another example of the Rosen type piezoelectric transformer mounted on a support member, and which substantially corresponds to those disclosed in JP-A-09-298213 and U.S. Pat. No. 5,747,916, and also with reference to FIG. 2B which is a diagrammatic sectional view taken along the line B--B in FIG. 2A but showing a condition that the piezoelectric transformer is supported within a housing. In FIGS. 2A and 2B, elements corresponding in function to those shown in FIGS. 1A and 1B are given the same reference numerals, and explanation will be omitted for simplification of description.
As will be seen from comparison between FIGS. 1A and 1B and FIGS. 2A and 2B, the piezoelectric transformer shown in FIGS. 2A and 2B is substantially the same as that shown in FIGS. 1A and 1B in construction and in operation principle, excluding that the support member 520 is constituted of a spring in place of the rubber.
Next, a heat-dissipating technology, which is important in a high-power piezoelectric transformer, will be described with reference to Japanese Utility Model Application Pre-examination Publication No. JP-U-06-82870. FIG. 3 is a diagrammatic perspective view illustrating the Rosen type piezoelectric transformer, disclosed in JP-U-06-82870, having a heat-dissipating plate mounted thereon for dissipating heat generated in the piezoelectric transformer.
The piezoelectric transformer shown in FIG. 3 is substantially the same as the piezoelectric transformers mentioned above in fundamental construction and in operation principle. Therefore, in FIGS. 3, elements corresponding in function to those shown in FIGS. 1A and 1B are given the same reference numerals, and explanation will be omitted for simplification of description.
The piezoelectric transformer shown in FIG. 3 is different from the piezoelectric transformers mentioned above in that, in order to dissipate heat generated in the piezoelectric transformer, the piezoelectric transformer shown in FIG. 3 has a heat-dissipating thin plate 806 which is formed of an aluminum thin plate and which is bonded by an epoxy resin bonding agent to a node where a heat generation becomes maximum.
In the above, the package structures of the Rosen type piezoelectric transformer have been described. In addition, a piezoelectric transformer having an operation principle different from that of the Rosen type piezoelectric transformer, has been proposed by for example Japanese Patent Application Pre-examination Publication No. JP-A-09-275231 (an English abstract is available and the content of the English abstract is incorporated by reference in its entirety into this application).
The piezoelectric transformer disclosed in JP-A-09-275231 is constituted by alternately stacking a plurality of piezoelectric ceramic layers and a plurality of internal electrode layers, and is so driven that the piezoelectric ceramic layers polarized in a thickness direction of the layers vibrate in a layer direction.
This laminated piezoelectric transformer has a high energy conversion efficiency, and by suitably selecting the number of stacked layers, it is possible to easily adjust the impedance.
In the prior art laminated piezoelectric transformer, however, a problem is encountered that it is difficult to package the piezoelectric body in an external housing without disturbing the vibration of the piezoelectric transformer.
Namely, it is desired to firmly couple the mechanically vibrating piezoelectric body to the external housing, but on the other hand, in order to effectively transmit the vibration of the driver section to the generator section, it is preferable that the piezoelectric body vibrates as freely as possible.
Therefore, a proposal to minimize attenuation of the mechanical vibration, has been made in which the support member constituting a connection point between the piezoelectric body and the housing, is mounted on a region of the node appearing in the vibration of the piezoelectric body. In addition, the support member is constituted of the elastic member formed of the rubber or the metal spring, similarly to the Rosen type piezoelectric transformer.
However, this proposal is effective in the Rosen type piezoelectric transformer, but cannot be applied to the laminated piezoelectric transformer having the different operation principle without modification. The reason for this is that:
In the laminated piezoelectric transformer, since the vibration node is a point, the piezoelectric transformer cannot be fixed in the housing in a stable condition highly resistive to vibration and shock, by use of the support member constituted of an elastic material such as the rubber and the metal spring. As a result, a relative positional deviation occurs between an upper supporting point and a lower supporting point, so that the piezoelectric transformer becomes inclined.
Furthermore, in the above mentioned proposed package structure for the laminated piezoelectric transformer, the heat generated for the piezoelectric vibration, which has become large particularly in a high-power piezoelectric transformer, cannot be effectively dissipated to an external. The reason for this is that:
In the laminated piezoelectric transformer, since the vibration node is a point, the heat-dissipating thin plate used in the Rosen type piezoelectric transformer cannot be used. In addition, since the piezoelectric transformer vibrates with a high frequency and with a large amplitude, a mechanically weak structure typified by the heat-dissipating thin plate is broken because of the vibration. Alternatively, the vibration of the piezoelectric transformer is transmitted to the heat-dissipating thin plate, so that noises are generated.